The act of switching radio frequency signals in an integrated circuit is carried out by an RF switch circuit. RF switches are well known in the art and provide a key building block in wireless systems. RF switches may be utilised in numerous applications such as mobile phones and wireless Local Area Networks (LANs). Such switches may include any number of switching elements which cooperate to control the flow of RF power between various circuit nodes. Performance metrics such as low insertion loss, high linearity, high isolation and power handling are critical in RF switch design.
Generally an RF switch does not consist of the RF switching circuit alone. Typically RF switch system is comprised of two domains; an RF domain which includes the switching elements and a DC domain which includes control logic, bias generation and power management circuitry. When the switch is operational, a high degree of isolation must be maintained between the RF and DC domains. Inadequate isolation between domains will compromise performance of the system resulting in reduced linearity, reduced power handling capability, reliability and operating lifetime may also be reduced.
Electrostatic discharge, (ESD), events can occur at any stage in the processing or handling of integrated circuits (ICs). The robustness of an IC to ESD is an important consideration that is determined by its capability to safely discharge a high current pulse generated during an ESD event without developing excessive voltage levels or heating that can cause damage to devices on the IC. There are various models to reproduce ESD events to which the IC may be subjected, for example, the Human Body Model (HBM) and Machine Model (MM). Both model an ESD event where discharge occurs between any two pins of an IC. Industry standard targets for IC ESD robustness are typically 1-2 kV for HBM and 100-200V for MM.
In the case of an RF switch, occurrence of an ESD event between a pin in the RF domain and a pin in the DC domain is particularly challenging. There is a conflict between the requirement to provide a low impedance path between the RF and DC domains in order to discharge current during an ESD event, and the requirement to maintain a high level of isolation between the RF and DC domains during normal operation.
The traditional approach to address this conflict has been to prioritise RF performance at the expense of ESD robustness by maintaining isolation between the RF and DC domains at die level. This approach originated from the historical arrangement where RF and DC sections were not integrated on the same semiconductor die. The RF domain was often a GaAs integrated circuit (IC) containing RF switch transistor elements, while the DC domain included a separate CMOS controller IC. As CMOS RF switches became more common, with RF and DC domains integrated on the same die, the existing approach for providing ESD protection at die level was maintained despite it being susceptible to inter-domain ESD events. In the traditional approach inter-domain ESD robustness of 1-2 kV is typically achieved when an external conduction path is provided between the RF and DC pins when both domains are connected to a common ground. Prior to providing the external conduction path ESD robustness of the die to a HBM event is typically limited to less than 100V. Dies with lower ESD robustness levels are more susceptible to yield fallout during packaging, test and handling and may also incur higher levels of product failures in field. Tighter ESD controls are required in facilities processing dies with low levels of ESD robustness, restricting the choice of location where these devices may be safely handled and processed.
There is therefore a need to provide an RF switch which addresses at least some of the drawbacks of the prior art.